Mutation is the fundamental agent of evolutionary change, and deleterious mutations have been implicated as the causal agent responsible for a variety of important biological phenomena. Recent developments have cast doubt on the prevailing wisdom regarding rates and average effects of new mutations at both the phenotypic and the molecular levels. Unfortunately, good data on mutational properties exist for only two species, Drosophila melanogaster and Caenorhabditis elegans, species that are phylogenetically distant and that differ substantially in life history. Almost nothing is known about how mutational properties vary over fairly small evolutionary distances. Herein I propose to investigate the mutational properties of three species of Rhabditid nematode. Mutations will be allowed to accumulate in initially genetically identical stocks for upwards of 100 generations. Standard methods will be used to estimate the rate and average effect of new mutations with detectable effects on the phenotype, and to partition the variance into within- and among-species components. Molecular methods will also be used to directly estimate the mutation rate at the level of the DNA sequence. The resulting data will allow the interpretation of differences and/or similarities in mutational properties among widely separated taxa in the context of dynamic evolution, that is, within and among closely related species.